At what stage of stomach cancer does fluid start to build up in the abdomen?

Advanced stage: Ascites or fluid in the abdomen develops in the cancer of stomach in late stages in the cancer of stomach due to invasion of peritoneum.

Usually Stage IV: In the setting of stomach cancer, fluid build-up - or 'ascites' - usually means that tumor cells have spread to the lining of the abdomen. These metastases interfere with the body's normal secretion and reabsorption of fluid, therefore allowing fluid to build up. Once tumor cells have spread to distant sites, the cancer is considered stage iv.

IV: If cytology is positive in the abdominal fluid, then it is stage iv. Hope this helps.